To help the reader understand the description of the invention, the following abbreviations are provided:
(e)SRVCC(enhanced) Single Radio Voice Call Continuity2G2nd Generation Mobile Network3G3rd Generation Mobile Network4G4th Generation Mobile NetworkAKAAuthentication and Key AgreementAPNAccess Point NameBSSBusiness Support SystemCAMELCustomized Applications for Mobile networks EnhancedLogicCSCircuit SwitchedCSFBCircuit Switched FallbackCSICamel Subscription IdentityDiameter CCADiameter Credit Control AnswerDiameter CCRDiameter Credit Control RequestGERANGSM EDGE Radio Access NetworkHLRHome Location RegisterHPMNHome Public Mobile NetworkHSSHome Subscriber ServerI-CSCFInterrogating Call Session Control FunctionIMPIIMS Private IdentityIMPUIMS Public IdentityIMSIP Multimedia SystemIMS-AGWIMS Access GatewayIMS-ALGIMS Application Level GatewayIMSIInternational Mobile Subscriber IdentityLBO-HRLocal Break-Out HPMN RoutedLBO-VRLocal Break-Out VPMN RoutedLTELong Term EvolutionMAPMobile Application PartMAP ATSIMAP ANY TIME SUBSCRIPTION INTERROGATIONMAP SAIMAP Send Authentication InfoMGCFMedia Gateway Controller FunctionMNOMobile Network OperatorMOMobile OriginatingMSISDNMobile Station International Subscriber DirectoryNumberMTMobile TerminatingMVNOMobile Virtual Network OperatorOCROptimal Call RoutingO-CSIOriginating Camel Subscription IdentityOMROptimal Media RoutingOSROptimal Signalling RoutingOSSOperations Support SystemOTTOver-The-TopPBXPrivate Branch ExchangeP-CSCFProxy Call Session Control FunctionPGWPacket GatewayIMS PSIIMS Public Service IdentityRATRadio Access TechnologyRAVELLocal Break-Out VPMN RoutedRCSRich Communicating SuiteR-LBORegional Local Break-OutRTCPRTP Control ProtocolRTPReal-time Transport ProtocolS8-HRS8 Home RoutedSBCSession Border ControllerSCC-ASService Centralization and Continuity Application ServerS-CSCFServing Call Session Control FunctionSIPSession Initiation ProtocolSSSupplementary ServicesT-ADSTerminating Access Domain SelectionT-CSITerminating Camel Subscription IdentityTRFTransit and Roaming FunctionTrGWTransition GatewayUEUser EquipmentUTRANUniversal Terrestrial Radio Access NetworkVoLTEVoice over LTEVoWiFiVoice over WiFiVPMNVisited Public Mobile NetworkVT-CSIVisited Terminated Camel Subscription Identity
Having launched Long Term Evolution (LTE) a few years ago, the next challenge for mobile service providers is to implement Voice over Long Term Evolution (VoLTE), which relies on the Internet Protocol Multimedia Subsystem (IMS) framework to deliver telephony services over LTE. In VoLTE the emphasis has been placed on Guaranteed Quality of Service which inherently translates into high definition (HD) calling, a service that consumers have come to expect from telephony applications due to the push from Over-The-Top providers (OTTs). Where VoLTE differentiates itself from OTT applications is in the way resources are guaranteed to assure a predictable and consistent service behavior to the consumer. OTT applications on the other hand are offered in a best effort manner, so if resources are scarce the quality of the call may be deteriorated.
The move to VoLTE is a long and complex process and service providers are only now shifting their focus to this technology. Up to now, voice services for devices connected to fourth generation mobile networks (4G) have been rendered through the use of Circuit Switch Fall Back (CSFB) in 3GPP networks. CSFB allows a service provider to re-use its legacy circuit switched infrastructure with minimal integration effort and investment compared to the implementation of a full-fledged IMS core. The drawback of CSFB is the device is forced to switch to 2G or 3G before it can make use of voice services. This means that call setup time is longer than for a pure 2G/3G call since a handover procedure that is part of the call flow and call quality is inferior to VoLTE and HD calling. As development of innovative services is now solely focused on LTE, CSFB is certainly not a viable solution for the future. While CSFB is a stopgap solution, operators eventually look forward to implement an IMS core for next generation telecom applications including VoLTE.
Finally, implementing an IMS core also paves the way for future services such as a Rich Communicating Suite (RCS).
Implementing VoLTE on top of an IMS core is not a trivial task. The IMS framework provides a generic infrastructure capable of supporting any type of multimedia services. The price to pay for this flexibility lies in its complexity to realize. An IMS core comprises of a plethora of network elements each exposing interfaces requiring integration based on Session Initial Protocol (SIP), Real-time Transport Protocol (RTP), or Diameter.
Moreover, the cost of investment is prohibitive. Although IMS promises to power future multimedia services, it is still unclear what those services are, except for VoLTE or how successful they will be in the future. Service providers are thus hesitant to invest resources towards a technology where the commercial viability is still unknown, except for its implementation for voice based services. This is where an international carrier can bring added value by introducing a centralized and multi-tenant IMS solution re-usable for any service provider and tightly integrated with the Evolved Packet Core (EPC) of the operator. This would eliminate the complexity of implementing an IMS core at each Mobile Network Operator (MNO) and would speed up time to market, ensuring service providers are not left behind.
The challenges faced by service operators remain the same, but the responsibility to overcome these obstacles is now transferred to carriers. For example,                carriers now need to integrate the IMS located centrally (or over the cloud) with the EPC of the MNO;        end users must be provisioned by the carrier at the IMS-HSS, which needs elaborate integration with the Operations Support System (OSS/BSS) systems of the MNO. The BSS must evolve accordingly to be compatible with VoLTE billing and rating. In case the IMS-HSS is at the MNO end, then the IMS core must have a Diameter (Cx/Sh) interface between the CSCF Application Servers (CSCFs/ASs) and the HSS; and        the solution must ensure that local traffic stays local, which can be realized by deploying a local SBC. However, there is no easy way to keep the signaling information for the local traffic confined within the MNO, unless there is a locally deployed IMS core.        
Moreover, in order to guarantee quality, privacy, security and flexibility towards customers, a carrier would need to deploy one IMS core per operator which is not practical from a technical and commercial point of view. This is the reason why hosted VoLTE services (hosted by the carrier network) are not particularly popular today. Moreover, it becomes uneconomical to implement this solution for small sized networks and enterprises, as cost and complexity of operations remains almost the same.
Early adopters of VoLTE, however, are faced with a different issue. Having made the investment, these service providers are now able to deliver VoLTE to their subscribers for national use and are eager to offer this service in roaming conditions. But traditional roaming models rely on an IMS core being present at the home and visited network. Given the aforementioned hindrances to introducing an IMS core within a service provider, early adopters are seeking solutions to sidestep the restrictions imposed by the traditional models.
One option is the S8 Home Routed (S8-HR) model where a VoLTE enabled network can capitalize on existing LTE roaming agreements to offer VoLTE services to its roaming out subscribers by forcing all Session Initial Protocol (SIP) and Real-time Transport Protocol (RTP) traffic back to its home IMS network through its home Packet Gateway (PGW). Charging and lawful intercept matters are the main downsides of this model. Moreover, the mouth to ear delay can be high, depending on the location of the Visited Public Mobile Network (VPMN) and Home Public Mobile Network (HPMN).
Another option is to implement a solution platform at the international carrier level which acts as a bridge between the LTE and VoLTE world. The model termed Regional Local Breakout (R-LBO) requires the carrier to implement a partial EPC (i.e., a PGW and PCRF) and a partial IMS core (i.e., a P-CSCF, IMS-ALG, IMS-AGW, IBCF, TrGW) and optionally TRF. With this architecture, the carrier will effectively enable a mobile user belonging to a VoLTE-enabled network to roam in an LTE-only network. The system in this case acts as a cloud bridge between an LTE-enabled visited network and a VoLTE-enabled home network. Billing and lawful interception services at VPMN can be provided by the carrier—services which are difficult to realize via the S8 Home Routing model. Such services at VPMN can also improve the mouth to ear delay of the call. The scope of the solution is only inbound roaming. Interworking and national operations cannot be rendered by the carrier, though a part of the VoLTE infrastructure is mandatory for the setup. This leaves room for reinvention aiming at optimization, broadening of functionalities, and facilitating a cleaner and greener solution.
To circumvent these generic problems faced presently by the mobile industry, it would be advantageous to propose a solution that enables a carrier or operator to deploy VoLTE rapidly for customers without the use of a full-fledged IMS core, thus making the best use of edge and cloud computing.